There are disclosed in co-pending patent application Ser. No. 08/800,727 filed Feb. 13, 1997, incorporated herein by reference, and co-pending PCT patent application Ser. No. PCT/US98/02566 filed Feb. 10, 1998, incorporated herein by reference, aqueous well drilling and servicing fluids which exhibit a high low shear rate viscosity (hereinafter often referred to as "LSRV") and which contain aphrons (i.e., microbubbles of a gas). The preferred fluids have the aphrons generated by imbibing air contacted by the fluids and creating the aphrons due to the pressure drop and cavitation which occurs when the flowing fluid exits the drill bit. However inert gases such as nitrogen and carbon dioxide can be incorporated into the fluids rather than relying on imbibed air or can be generated in-situ by reactive components such as carbonates and acids. Moreover, the aphrons can be generated at the surface of the well and incorporated into the fluid or the aphrons can be created at the surface in the fluid.
This invention relates to aphron-containing well drilling and servicing fluids wherein the fluids have an oleaginous liquid continuous phase.
Horizontal wells drilled and completed in unconsolidated sand reservoirs have become feasible recently, due to new technology and completion methods. Wells of this type require sand control, for example such as long open hole gravel packs or the installation of mechanical sand exclusion devices (slotted liners, prepacked screens, etc.). Successful wells have been completed with horizontal techniques, producing intervals as long as 1800 ft (550 m) and more using these methods of sand control.
Usually the wells are drilled with conventional drilling muds to the top of the pay zone and casing is set. The cement is then drilled out to the casing shoe and the shoe is tested. The drilling mud is then displaced with a "low damage potential drilling fluid" generally consisting of polymers, viscosity enhancers and particles for building a filter cake. The particles are usually graded salt (NaCI) or graded calcium carbonate (CaCO.sub.2). These compounds are used because they are soluble in undersaturated brines or hydrochloric acid.
After the open hole interval has been drilled to total depth, the gravel pack screen or sand exclusion device is placed in the open hole interval. To do this it becomes necessary to circulate the drilling fluid from the open hole so that the well can be gravel packed or the sand exclusion setting can be tested. Displacement of the drilling fluid with a solids-free completion brine is necessary. Concern about the physical erosion of the filter cake with the completion fluid is also always an issue. That is, the filter cake should be durable and stable enough to permit the completion or other operation to take place and protect the well bore during the entire operation.
Drilling of microfractured shales, microfractured and vugular carbonate and dolomite formations requires a drilling fluid which will seal these formations preventing the loss of gross amounts of fluids to the formations.
The ideal drilling mud or drill-in fluid would mechanically seal all pore openings, microfractures, and the like exposed to the wellbore, stay intact during completion operations, then be easily removed by production of oil or gas. Problems arise in designing these fluids or muds because production zones vary in pressure, permeability, porosity and formation configuration. It would be desirable if fluids could be devised which would prevent the loss of expensive completion fluids to the formations and which effectively protects the original permeable formation during various completion operations such as gravel packing or well bore workovers.
Oil muds and invert emulsion (oil base) drilling fluids have found application where the use of water-based fluids would result in damage to the formation through which the drilling is progressing. For example, it is known that certain types of shale will heave and collapse if water-based drilling fluids are used. Since the oil-based drilling fluids do not result in any swelling of the shale, their use circumvents the heaving problem. Invert emulsion muds basically contain an oleaginous medium, such as hydrocarbon liquid as the continuous phase, water as the dispersed phase, various emulsifying agents, wetting agents, weighting agents and viscosifiers, such as amine treated clays.
One of the disadvantages of oil base muds is their tendency to promote lost circulation during drilling as compared to water base muds of the same density.